1. Field of the Invention
This invention relates to a method of depositing epitaxial layers on monocrystalline substrates and, more particularly, to a method of depositing silicon epitaxial layers on silicon wafers.
2. Description of the Prior Art
Epitaxial semiconductor materials, chiefly silicon, are widely used for the fabrication of bipolar integrated circuits and discrete/power devices. Recently, epitaxial silicon has also been applied to the fabrication of MOS devices for eliminating some serious circuit performance limitations, such as latch up in CMOS structures and soft errors in dynamic RAMs. Epitaxial silicon can significantly enhance circuit speed and improve MOS yield. Therefore, the silicon epitaxy has become an important technology in semiconductor industry. However, common defects occuring in silicon epitaxial layers are slip, dislocation, stacking fault, and saucer. The slip and dislocation defects are well known to be sinks for impurities, and to be the cause of diode leakage and emitter-collector short to deteriorate the performance of the devices fabricated from such wafers. Thus, the elimination or reduction of the defects has become an important object in the epitaxial process.
In a radio-frequency heating reactor, the substrate is heated only from the back-side by the susceptor causing an axial temperature gradient across the substrate. The front-to-rear temperature difference causes a differential expansion of the substrate. In effect, the substrate curls up on the susceptor surface. As a result of this, the temperature at the peripheral edge of the substrate will drop causing a further reduction in comparison to central portions of the substrate. These radial temperature gradients will produce sufficient internal stresses to create dislocations and slip lines in the substrate. In general, the slip lines and dislocations follow a typical pattern, originating at the perimeter and extending toward the substrate center. Furthermore, defects generated in the substrate will propagate through the deposited layers. The presence of these defects affects the quality and reduces the yield of the devices produced from such epitaxial wafers.
The conventional method for solving the problem of slip lines and dislocations is to put the substrate into a flat-bottom shallow recess on the upper side of the susceptor. The depth and size of the recess correspond approximating to the thickness and size of the substrate. In this manner, the side-wall of the recess can supply radiation energy to the peripheral edge of the substrate to reduce the radial temperature gradients in the substrate. The recess can partly reduce the density of defects. However it cannot inhibit the generation of slip lines and dislocations completely. Furthermore, several approaches for reducing the thermal gradients in the substrate had been provided which include flat-bottom depressions on the upper side of the graphite heater(U.S. Pat. No. 3,436,255 1969), recesses on the lower side of the susceptor(U.S. Pat. No. 3,892,940 1975), an encircling ring around the rim of the substrates(U.S. Pat. No. 4,113,547 1978), and a spherical depressions on the upper side of the susceptor(J. Electrochem. Soc. 129, 2858, 1982). Those approaches did suppress the generation of slip lines and dislocations to a minimum, but had not given slip-free and dislocation-free epitaxial growth in a radio-frequency heating reactor.
Accordingly, the object of this invention is to provide a simple and highly efficient method of reducing the thermal gradients in the substrate to a minimum and giving slip-free and dislocation-free epitaxial growth in a radio-frequency heating reactor.